1. Field of the Invention
The present invention relates to a semiconductor strain sensor that includes a piezoresistive element and an electrode for externally extracting a signal from the piezoresistive element on a silicon-on-insulator (SOI) substrate.
2. Description of the Related Art
Conventional semiconductor strain sensors include: a semiconductor substrate such as single-crystalline silicon, etc., in which are formed: a thin diaphragm portion; and a thick outer frame portion that is positioned around the diaphragm portion; and a glass pedestal that is joined to the outer frame portion of the semiconductor substrate (see Patent Literature 1, for example). A plurality of strain gauge resistors in which resistance changes due to piezoresistance effects when pressure is applied are disposed on the diaphragm portion so as to constitute a bridge circuit, and pressure is detected by detecting changes in the resistance of the strain gauge resistors as changes in voltage. Electrodes that are made of aluminum, etc., are connected to the strain gauge resistors by wiring so as to output pressure signals externally.
[Patent Literature 1] Japanese Patent Laid-Open No. 2002-131161 (Gazette)
In conventional semiconductor strain sensors, aluminum is used in the electrodes, and single-crystalline silicon is used in the semiconductor substrate. The thermal expansion coefficient of aluminum is approximately 23.5 ppm/° C., and the thermal expansion coefficient of single-crystalline silicon is approximately 2.5 ppm/° C. Thus, because the thermal expansion coefficients of the electrodes and the semiconductor substrate differ greatly, one problem has been that the electrodes and the semiconductor substrate may warp if the ambient temperature at which the sensor is used changes, generating thermal stresses and giving rise to temperature drift, or thermal hysteresis, etc., in the output signals from the sensor.